The goal of this investigation is to determine if altered host defenses explain the increased vulnerability of the elderly to infection. Humoral and cellular immune responses, phagocytic function by peripheral leukocytes, leukocyte migration in vivo, neutrophil leukotactic responses in vitro and the prevalence of auto-immune responses in a group of individuals 60 years of age or greater and in good health will be evaluated. BIBLIOGRAPHIC REFERENCE: Phair et al: Humoral Factors and Leucocyte Function in the Aged, Clin. Res. 23: 529A, 1975.